Background: Tryptophan side chains can influence the binding of amphipathic peptides to biological membranes. <p>Results: The cytotoxic activity of model helical amphipathic peptides was markedly influenced by the positions of tryptophan residues in the sequence. <p>Conclusion: Tryptophan residues located adjacent to a hydrophobic helical portion created the most potent cytotoxic peptides. <p> ...